Glycol monoesters are compounds having the following structure, where R.sup.1 and R.sup.2 are each either hydrogen or an organic group (e.g., an alkyl group): ##STR1##
These compounds have a variety of uses. For example, glycol monoesters are used as industrial intermediates, solvents, plasticizers for polyvinylchloride and polyolefins, and coalescing aids for latex paints. One well-known glycol monoester is a product from 2-methyl propionaldehyde, where both R.sup.1 and R.sup.2 in the above formula are methyl groups, which is sold under the TEXANOL.RTM. trade name by Eastman Chemical Company, Kingsport, Tenn. The TEXANOL.RTM. product is the premier coalescing aid and leveling agent for latex paints and is also used as a plasticizer for polyvinylchloride products.
The formation of glycol monoesters from aldehydes is well known in the art. A typical process for forming glycol monoesters begins with an aldol condensation of an aldehyde having an alpha-hydrogen followed by a Tischenko hydride shift. Scheme I depicts these reactions, including the formation of a dioxanol intermediate. ##STR2##
Although the glycol monoester above depicts the ester in the 1-position, the glycol monoester actually exists as a mixture of the 1- and 3-isomers. These isomers interconvert through an equilibrium (shown in scheme II) at sufficiently high temperature, in the presence of a catalyst, or over time. ##STR3##
Under conditions used in most glycol monoester manufacturing processes, competing side reactions result in numerous undesirable by-products. Moderately high temperatures, for instance, favor transesterifications which yield glycol and glycol diester by-products. Simple ester by-products are believed to occur through either an intramolecular Tischenko reaction, (shown in scheme III), or through an intermolecular Cannizzarro reaction, (shown in scheme IV). High temperatures favor Cannizzarro reactions producing a simple ester by-product to the exclusion of the aldol condensation products. As a consequence of these unwanted by-products, the aldehyde conversion must be regulated to low to moderate levels to give optimum yields of glycol monoester. Even then, the desired glycol monoester yield rarely exceeds 85 to 90 percent without extensive recycling of the transesterification products. ##STR4##
Strong bases have traditionally been used to promote the formation of glycol monoesters from aldehydes. For example, Hagemeyer et al., U.S. Pat. No. 3,081,344, describes how moderately basic aluminum alkoxides are good Tischenko catalysts, but poor aldol catalysts. Hagemeyer et al., U.S. Pat. No. 3,091,632, describes how highly basic sodium alkoxides are good aldol/Tischenko catalysts, but are poor Cannizzarro catalysts except at higher temperatures. The difference in catalytic base strength in these two patents accounts for the difference in the relative amounts of the simple ester product and the glycol monoester product. Both form from hemiacetals undergoing hydride attacks on carbonyl carbon atoms. The simple ester product forms from a hydride transfer from a hemiacetal of the starting aldehyde, and the glycol monoester forms from a hydride shift in the hemiacetal of the starting aldehyde and its aldol condensation product.
Additional by-products in the preparation of glycol monoesters are glycol, glycol diester and alcohol. These by-products result from transesterifications, as shown in schemes V-VII: ##STR5##
The extent of transesterification is generally related to the temperature and the reaction conditions for the aldehyde conversion. Under low simple ester production conditions, the limiting values of the transesterification keep the glycol:glycol monoester:glycol diester ratio near 1:2:1 so that the maximum yield of the glycol monoester is 50 percent. At high simple ester production conditions, the glycol:glycol monoester:glycol diester ratio becomes skewed toward the glycol diester as the simple ester transfers its ester group to the glycol derivatives releasing the alcohol. At high simple ester production conditions coupled with the limiting transesterification conditions, the yield of the glycol monoester may fall significantly below 50 percent. Even when the transesterification is not extensive, such as at low reaction temperatures and low aldehyde conversion rates, the yield of glycol monoester rarely exceeds 85-90 percent. The maximum yield of glycol monoester for the aqueous caustic system ranges from 75 percent, as shown by German Patent No. 3,447,029 to Weber et al. (1986), to 92 percent at moderate aldehyde conversions, as shown by Swedish Patent No. 468,559 to Hopfinger et al. (1993), to 98 at low aldehyde conversions percent, as shown by Japanese Patent No. 77 15,582 to Tsuchiya et al. (1977).
Because significant amounts of by-products often result, processes to prepare glycol monoesters require elaborate and costly purification schemes to separate the product from the by-products and to reconvert the by-products into glycol monoesters. Processes requiring over one dozen distillation columns and two or more reactors are not uncommon. Thus, a need exists for an improved glycol monoester synthesis. A further need exists for a catalyst that promotes aldol condensations and Tischenko hydride shifts to produce glycol monoesters while minimizing unwanted by-products.